Existing LEDs can emit light in the ultraviolet (“UV”), visible or infrared (“IR”) wavelength range. These LEDs generally have narrow emission spectra. It is desirable to use LEDs that can generate broader emission spectra to produce desired color light, such as white light. Due to the narrow emission spectrum, LEDs can not be directly used to produce broad-spectrum light. Phosphors can be introduced to convert portion of the light originally emitted by the LED into light of a different wavelength. The combination of the converted light and the originally emitted light renders a more desired output light. However, phosphors generally have narrow absorption spectra and can be only used on underlying light sources that have a very specific range of emission wavelengths. For example, the YAG:Ce phosphor is optimized for 460 nanometer (“nm”) light but is not suitable for LEDs emitting light at other wavelengths. The white light converted by one type of phosphor typically has a low color rendering index (“CRI”) and can only reach a limited range of color temperature.
Quantum dots (“QDs”, also known as semiconductor nanocrystals) can be used to convert the light emitted by LEDs and to generate the light in the visible or infrared region. Quantum dots are small crystals of II-VI, III-V, IV-VI materials that typically have a diameter between 1 nm and 20 nm, which is smaller than the bulk exciton Bohr radius. Due to the quantum confinement effects, the energy differences between electronic states of a QD are a function of both the composition and the physical size of the QD. Thus, the optical and optoelectronic properties of QDs can be tuned and adjusted by changing the physical size of the QDs. The QDs absorb wavelengths shorter than the absorption onset wavelength and emit light at the absorption onset wavelength. The bandwidth of the QD luminescent spectra is related to temperature dependent Doppler broadening, Heisenberg Uncertainty Principle and the size distribution of the QDs. For a given QD, the emission band of the QD can be controlled by changing the size. Thus, the QD can produce a range of colors that are unattainable with conventional phosphors. For example, CdSe QD of 2 nm emits at the blue region and CdSe QD of 10 nm emits at the red region.
Due to the nano-scale size of the QDs, the QDs conversion layer is intrinsically a non-scattering layer. With lack of scattering, light has a much shorter optical path when it passes through a QDs layer than when the same light passes through a conventional phosphor layer. Unless the layer thickness is undesirably high, not enough light will be absorbed by the QDs to convert into a desired amount of emission at another wavelength. Therefore, an undesirably thick QD layer or high quantity of QDs would be needed to achieve the target color combination. Moreover, the optical path length is different at different angles of emission. For a QDs layer to convert light into the red region, the light at a smaller angle of emission has a shorter optical path and less red light is generated, meanwhile the light at a larger angle of emission results in more red light. Thus, the resulting output light has less red component in the center and more red component at the edge, which is disadvantageous in terms of color uniformity.
The following are known: U.S. Pat. No. 6,890,777 (Bawendi); US Pat. Pub. Nos. 2008/0173886 (Cheon), 2007/0246734 (Lee), 2010/0123155 (Pickett), and 2007/0221947 (Locascio); and the article “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection”, Xi et al., Nature Photonics, Vol. 1 pp. 176-179 (pub'd online 1 Mar. 2007).